This invention pertains generally to the field of induction heating devices, particularly, to side entry coil induction heaters that are used to heat a load, and more particularly are used to form blocks in wire harness and cable assemblies.
In fabricating cables and harnesses containing a plurality of wires it is desirable to provide fluid blocks to prevent the passage of fluids, such as water, along the cable. This problem arises in various industrial and commercial applications where cables are used, such as the automotive and telecommunications fields. In cable assemblies used in automobiles, for example, it is important to prevent moisture from migrating along the wires in the cable to various electrical components in different parts of the car. It is also desirable to avoid the passage of fumes and noise through the cable from the engine compartment to the passenger area.
Various techniques have been employed to deal with this problem. In the automotive field, wire harness assemblies are sometimes arranged with "drip loops", which consist of U-sections of the wires hanging down so that water passing along the wires will drip off at the bottom of the U-section. Obviously this is only a partial solution to the problem.
A desirable technique is to provide a packing or sealant around the wires in a protective sleeve, which is designed to form a complete fluid block. This technique is described in detail in U.S. Pat. No. 4,972,042 ("Blocking Arrangement for Suppressing Fluid Transmission in Cables"), issued to Seabourne et al. on Nov. 20, 1990, assigned to the same assignee as the present application and incorporated herein by reference. This patent discloses the use of fusible polymeric sealants, such as hot-melt adhesives or thermosetting adhesives, in a heat-shrinkable covering or sleeve surrounding the cable wires. The application of heat to this assembly causes the adhesive to melt and surround the wires, forming a block upon cooling. Epoxy sealants may also be utilized, in which the application of heat facilitates curing and formation of a permanent fluid block in the cable.
This technique requires the application of heat to the assembly in a controlled manner, to provide a satisfactory blocking structure. Both the temperature of the assembly and the heating time must be carefully monitored. Excessive temperatures can cause damage to the cable wires or insulation, as well as the protective covering and sealant. On the other hand, if the heating temperature is too low, the blocking seal may not form completely and the block will be ineffective to prevent fluid passage. Ideally the heating should be uniform throughout the cable block to avoid hot spots and cold spots in the sealant.
Induction heating is a widely used heating method for applications requiring precise heating control. Although originally this method was developed primarily for heating metals, it has also been used for other materials. For example, U.S. Pat. No. 5,378,879, entitled "Induction Heating of Loaded Materials", issued on Jan. 3, 1995 to Y. Monovoukas, which is assigned to the same assignee as the present application and incorporated herein by reference, describes the induction heating of non-magnetic, electrically non-conductive materials by means of loading with suitable particles. As disclosed in that application, this technique may be used in the fabrication of sealant blocks in wire cable and harness assemblies.
Ideally, a simple induction coil of the usual solenoidal configuration would provide a uniform magnetic field and, therefore, uniform heating of the sealant, if the cable were disposed along the axis of the coil. However, this configuration is not suitable for normal manufacturing operations because it requires that the cable be threaded through the coil, which is a serious fabrication constraint. For practical purposes, the induction coil must have a shape that allows the coil to be brought close to the cable from the side, or laterally, at the location along its length where heating is desired without having to thread the cable through the coil.
Induction coils that provide such lateral access to the heating area have been designed with a variety of configurations and may be broadly described as side entry coil assemblies. One example of a side entry coil assembly is a U-shaped coil. Another example is actually constitutes two flat coils (or "pancake coils") located on opposite sides of the heating area, with the planes of these coils in parallel alignment. The currents in both coils circulate preferably in the same direction, to optimize the magnetic induction in the heating area.
For heating loads of elongated shape, such as cables, a particularly suitable side entry assembly configuration is the "channel coil" (or "U-channel coil"). A channel coil configuration may be obtained by taking a flat coil and deforming the plane of the coil into a "U-shape" about an axis that is parallel to the plane of the coil. Such a coil allows lateral access to the cable assembly, in that it forms a channel along which the cable can be laid through the opening in the "U". An important characteristic of this type of configuration is that in the central region along the channel, the magnetic field direction in the channel interior and mouth of the "U" is primarily transverse; that is, the direction is perpendicular to the channel axis.
Obviously, this type of channel coil does not have the degree of cylindrical symmetry provided by a solenoidal coil, and generally the magnetic field produced by a channel coil is highly non-uniform in the channel. Even if the field strength is relatively constant along the longitudinal dimension in the heated portion of the cable, unless the transverse dimensions of the cable are inordinately small, this implies that the magnetic field and the induction heating produced by the coil will not be uniform across the cable. This problem has been encountered in using channel coils to fabricate cable blocks using induction heating. The magnetic field is generally stronger near the base of the channel, and weaker near the opening of the "U". It has been found that when channel induction heating coils are used in this way, hot spots tend to form in the side of the cable near the U-base, while cold spots form near the opening of the "U". The unevenness of the heating is often manifested by lumps and voids in the sealant, and damaged insulation around the wires. In some samples, some of the wires near the U-base are found to become overheated and damaged, while in the same sample the sealant near the opening of the "U" fails to heat sufficiently to form a complete seal.